Mycobacterium avium-intracellulare (MAI) is an atypical mycobacterium commonly found in the environment which rarely causes invasive disease in humans in the absence of a pre-existing condition. In patients with the Acquired Immunodeficiency Syndrome (AIDS), where the immune response is severely depressed, a disseminated form of MAI is found in over 50% of the patients. Thus, high resistance to this opportunistic pathogen must be well-developed in normal individuals, though mechanisms have not been established. We have recently found that natural killer (NK) cells and interleukin 2 (IL 2)-activated killer (LAK) cells from nonimmune donors can lyse autologous MAI-infected monocytes, thus limiting the intracellular growth of this organism. In this proposal, we suggest that NK/LAK cells may be involved in normal host resistance to MAI, and that cytokines released from MAI-stimulated NK cells can either activate infected monocytes to kill intracellular MAI or protect uninfected monocytes from invasion by this bacterium. Monocytes will be obtained from normal donors and infected in vitro with various ratios of MAI to determine the effect of intracellular growth of the bacterium on host cell viability, via trypan blue exclusion, and on the susceptibility of infected monocytes to lysis by NK/LAK cells, using a 4 hr 51 Cr-release assay. To determine if cytokines play a role in host resistance to MAI, interferon-alpha (IFN- alpha), IFN-gamma, tumor necrosis factor, interleukin 1, and IL 2 will be assessed for their ability to activate infected monocytes to kill intracellular MAI. Additionally, monocytes will be treated with these cytokines prior to inoculation with MAI to determine whether they can be protected from infections. Cytokines will also be assessed for their ability to modulate LAK-mediated lysis of infected monocytes by: 1) Treatment of monocytes to determine the effect of cytokine on susceptibility to lysis, and 2) inclusion on cytokines during IL 2-activation of effector cells. Monoclonal antibodies to MAI antigens will be raised and added to cytolytic assays to determine whether bacterial antigens on the surface of infected monocytes serve as target structures for LAK cells. Finally, the direct effects of NK/LAK cells on bacterial growth will be determined by co-culturing MAI with effector cells and counting residual bacteria after incubation. From these studies, we hope to further define the role of NK/LAK cells and cytokines in MAI infections, the study of which might lead to better treatment modalities for this usually drug-resistant bacterium.